The state of the art of implantable pulse generators for stimulating human tissue has advanced to the point that such devices are being designed and used in increasing numbers to treat a wide variety of medical conditions. In addition to implantable pulse generators for treating many different types of cardiac conditions (bradycardia, tachycardia, fibrillation, and the like), so-called neurological pulse generators have been provided for stimulating a patient's nervous system, in order to treat such diverse conditions as pain, motor impairment, incontinence, and impotence, to name only a few.
In most cases, electrical stimulation pulses are conveyed from an implanted pulse generator to the desired stimulation site by means of an implanted lead having exposed electrodes at its distal end. In order to achieve the desired effects from delivery of stimulating pulses it is of course very important that the lead be properly positioned and stabilized in the patient, so that as much of the stimulating energy as possible is delivered to the appropriate site. While this is true for all kinds of stimulation pulse therapies, lead positioning is especially critical in the area of neurological pacing, such as when stimulation pulses are delivered by a lead positioned in the epidural space adjoining the patient's spinal column. The delicate and highly sensitive nature of the spinal column, and the possible harmful or otherwise undesirable effects of delivering stimulation pulses to an inappropriate site in this area accentuates the need for accurate control in the guidance and positioning of the lead in order to achieve precise lead placement.
Stylets are commonly used in the field of neurological stimulation as a means for guiding and properly placing leads. Examples are disclosed in U.S. Pat. Nos. 4,285,347 to Hess and 4,419,819 to Dickhudt et al. Disclosed are neurological leads which utilize stiff wired stylets which provide the stability to the lead which is necessary to enable the lead to be inserted into the body through a Touhy needle and be guided in the body to the area of desired placement. In both cases the stylets are also used to retract anchor or stabilizing mechanisms while the lead is being inserted. In both leads the stylet is freely moveable within the lead at all times. Thus, although advancement of the stylet may advance the lead, withdrawal of the stylet does not result in a similar withdrawal of the lead.
Neurological leads which utilize stylets for guidance are also subject to the problem of lead twisting or torquing during placement. When the lead is being implanted it is common to use a stylet which is bent at an angle at the distal end in order to impart a similar bend to the distal end of the lead. Such a bend allows the lead to be manipulated around curves or corners. However, when these curves or corners are negotiated it may be necessary to rotate the lead. This can result in the lead rotating with respect to the stylet and creating rotational forces (torque). This creates a problem since, at some point, these forces will be released resulting in the lead twisting back to its normal position and quite possibly mis-aligning the lead.
In the field of cardiac pacing leads the use of stylets to guide the lead through a vessel to the inside of the heart is well known and very important to the proper placement of the lead. An example of such a lead is disclosed in U.S. Pat. No. 3,902,501 to Citron et al. In this lead the stylet is also used to release tines at the distal end in order to anchor the lead. In U.S. Pat. No. 4,913,164 to Greene et al. a pacing lead is disclosed in which a stylet is used to engage tines to anchor the lead. In one embodiment the distal end of the stylet may be threaded into an internal cup which allows a plunger to be drawn back by the stylet which results in the tines being withdrawn. U.S. Pat. No. 4,957,118 to Erlbacher discloses a pacemaker lead with an actuating rod permanently inserted in the lumen of the lead. The actuating rod is used to actuate tines at the distal end of the lead and also to conduct electrical signals to the tip of the lead. There is no provision in any of these leads for the use of a stylet for manipulating the insertion, positioning or withdrawal of the lead.
In U.S. Pat. No. 4,498,482 to Williams a transvenous pacing lead having a stylet with a ball shaped distal tip is disclosed. The tip of the stylet is soft for the purpose of allowing it to comply with curvatures and bends in the pacing lead. A similar ball tipped stylet is disclosed in U.S. Pat. No. 5,003,992 to Holleman et al. In the lead disclosed the ball tip of the stylet is inserted past a retainer ring in order to engage the stylet and allow the stylet to advance or retract an electrode at the distal tip of the lead. The stylet is used during placement of the lead only to keep the distal electrode retracted.
The problem with these leads is that they provide for use of the stylet only to stiffen the lead during its advancement. The stylet is not fixed with respect to the lead body to allow the stylet to be used to control lead advancement or retraction used in optimizing the position of the lead. These leads also make no provision for solving the problem of lead torquing.
In order to overcome the problems associated with present leads as set forth above it can thus be seen that there is a need for a lead having a stylet which is releasably secured with respect to the body of the lead. This provides the stylet with enhanced control during placement of the lead and stabilizes the lead body with respect to the stylet to overcome the problem of torquing.